Various types of so-called denitration catalysts and denitration methods, which reduce and remove nitrogen oxides in flue gases by causing the nitrogen oxides to contact a reductant such as ammonia in the presence of a catalyst, are known. Among such denitration catalysts, a honeycomb-shaped catalyst (honeycomb catalyst) constitutes a fixed bed of a catalyst reactor having through holes arranged in the direction of flow of the flue gas, and thus, if a honeycomb catalyst is used, less pressure loss by gas flow would occur, and as a result, the linear speed of the flue gas can be controlled to be higher compared with that in a catalyst reactor in which a fixed bed catalyst having a shape other than the honeycomb shape is charged. Further, when a honeycomb catalyst is used, if the flue gas includes dust, the through holes are less likely to be clog by the dust, and thus, gas catalytic reactions can be efficiently run. With the above-described advantages, honeycomb catalysts have been widely used for denitration reactions of combustion flue gases including nitrogen oxides that are generated from boilers and the like.
However, even if a honeycomb catalyst is used, if the honeycomb catalyst is used for treating a flue gas including extremely hard dust particles such as of calcium, magnesium, silica, alumina, iron, and the like, the dust particles may collide with one another, and the honeycomb catalyst may be gradually worn by the collisions of the dust particles, and thus, the honeycomb catalyst may finally lose its shape and may not withstand further use in some cases.
In order to prevent a honeycomb catalyst from wear, a method in which the strength of a honeycomb catalyst is increased by firing the front end thereof including an inlet for flue gas, and a method in which a honeycomb catalyst is strengthened by covering the surface thereof with a glass substance and thereby the wear resistance is increased have been proposed (e.g., Patent Literatures 1 to 3). Wear on the front end of a honeycomb catalyst occurring due to dust particles can be alleviated by strengthening the front end.
However, a flue gas emitted from a coal-fired boiler may include a large amount of high-hardness dust particles, depending on the type of the coal used in the boiler. If a large amount of dust particles is present, even if a honeycomb catalyst with a strengthened front end is used, the honeycomb catalyst may be locally worn in portions different from the front end.
In addition, if a denitration apparatus is to be additionally installed in a plant, the catalyst may not be allowed to have a sufficiently large section due to the limited installation area for the facilities. Accordingly, if a method in which a combustion flue gas is treated at a high flow rate such as flue denitration is used, the honeycomb catalyst may be more considerably worn.